1. Field of the Invention
The present invention relates to a method of producing grain oriented silicon steel sheets each having a very low watt loss. More particularly, the present invention relates to an improvement of the method of producing grain oriented silicon steel sheets each having a very low watt loss, wherein a watt loss property of each silicon steel sheet can be remarkably improved by a smooth and flat finishing of surfaces of each silicon steel sheet at a high operational efficiency.
2. Description of the Related Art
As is well known, many grain oriented silicon steel sheets are practically used as a raw material for magnetic cores incorporated in various kinds of electric devices, apparatus or the like, and in order to reduce energy loss, there is a demand by users for grain oriented silicon steel sheets each having a low watt loss. In this connection, a means for reducing a watt loss of each grain oriented silicon steel sheet is disclosed in an official gazette of, e.g., Japanese Unexamined Patent Publication Patent (Kokai) No. 58-26405, which is concerned with a method of reducing a value indicating a watt loss wherein a laser light beam is irradiated on one surface of a grain oriented silicon steel sheet after a completion of a finish annealing operation to induce a local strain on the silicon steel sheet, to thus cause a magnetic domain subdivisional treatment to be conducted. In addition, magnetic domain subdivisional treating means which ensure that a magnetic domain subdivisional treatment effect does not disappear, even when grain oriented silicon steel sheets are subjected to strain-remove annealing (stress-relief annealing) after being worked to a shape corresponding a core, is disclosed in an official gazette of, e.g., Japanese Unexmained Patent Publication (Kokai) No. 62-8617. It has been found that a watt loss of each grain oriented silicon steel sheet can be substantially reduced by employing any one of the aforementioned technical means. When a value indicating a watt loss of each silicon steel sheet must be further reduced, glassy films remaining on the surface of the silicon steel sheet after a completion of a finish annealing operation must be removed therefrom, and moreover, a roughness of the surface of a ferrous substrate of the silicon steel sheet effective for inhibiting a displacement of a magnetic domain on the surface of the silicon steel sheet must be removed therefrom. To this end, the surface of a ferrous substrate of each silicon steel sheet must be finished to a mirror surface after the completion of the finish annealing operation.
Additionally, a method of finishing the surface of a ferrous substrate of each steel sheet with a mirror surface, after the completion of a finish annealing operation, is disclosed in an official gazette of Japanese Unexamined Publication Patent (Kokai) No. 64-83620, which is concerned with a method of bringing the foregoing surface to a mirror surface by employing a chemical polishing process or a mechanical polishing process.
A chemical polishing process, an electrolytic polishing process and a mechanical polishing process conducted with the aid of a grinding wheel, a brush or similar means are used as a means for finishing the surface of a steel sheet to a mirror finish. The chemical polishing process and the electrolytic polishing process are preferably employed as a means for preparing a small number of test pieces, and cannot be employed as means for finishing the surface of a strip of metallic material to a mirror surface, e.g., a strip of silicon steel sheet produced on an industrial basis on a mass production line, because complicated operations for controlling the concentrations of various kinds of liquid chemicals and temperatures at various locations must be performed, and moreover, an expensive apparatus for preventing an occurrence of public pollution must be installed. Where the mechanical polishing process is employed, it is very difficult to uniformly finish a mirror surface of a metallic material having a large surface area, e.g., a strip of steel sheet produced on an industrial basis on a mass production line.